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The original JA Purity template was provided from JoomlArt.com as a zip but the latest files have been installed along with Joomla. The relevant files, header1.jpg, header2.jpg and header3.jpg, are in the following directory:

All you need to do is change the size of the file to match the height of your header image and be 2 pixels wider than your header images. The default mask size is width: 602 pixels, height: 80 pixels. So if you changed your header image to be 400 by 141 then the mask image would be 402 by 141. The mask can be recreated using a simple gradient tool in any graphics editor. Set the color of the gradient to match the color of the header.